Radiation is commonly used in the non-invasive inspection of objects such as luggage, bags, briefcases and the like, to identify hidden contraband at airports and public buildings. The contraband may include hidden guns, knives, explosive devices and illegal drugs, for example. One common inspection system is a line scanner, where the object to be inspected is passed between a stationary source of radiation, such as X-ray radiation, and a stationary detector. The radiation is collimated into a fan beam or a pencil beam. Radiation transmitted through the object is attenuated to varying degrees by the contents of the luggage. The attenuation of the radiation is a function of the density of the materials through which the radiation beam passes. The attenuated radiation is detected and radiographic images of the contents of the object are generated for inspection. The images show the shape, size and varying densities of the contents.
One disadvantage of radiographic imaging is that all items within the object in the path of the radiation beam are superimposed on the image. If there are many items in the object, it may be difficult to distinguish among them. In addition, the ability to identify a suspect item within an object may be dependent upon the item's shape and orientation within the object. in sheets of explosive materials may also be difficult to identify on a radiograph, particularly if they are oriented perpendicular to the scanning beam.
Computed tomography (“CT”) enables the reconstruction of the cross-sectional images of luggage contents, facilitating the identification of the items in the luggage. Since images are acquired at multiple angles, item shape and orientation are of less concern. CT images also provide higher resolution, greater image contrast and greater sensitivity to characteristics of the object being scanned, than radiographs. However, reconstruction of CT images of an object requires a large number of scans of the object at a plurality of angles. Conducting a sufficient number of scans for CT reconstruction is time consuming. Depending on the system used, CT imaging of an entire piece of luggage may be too slow for practical use in screening luggage in airports, for example.
Third generation CT configurations, where an X-ray source and a detector are mounted on opposite sides of a rotating gantry, have been used to scan luggage. The luggage is moved through the gantry and the X-ray source and the detector are rotated around the luggage. Examples of third generation CT systems for examining luggage are described in U.S. Pat. No. 5,567,552 and U.S. Pat. No. 6,078,642, for example.
The inspection of cargo containers at national borders, seaports, and airports is a critical problem in national security. Due to the high rate of arrival of such containers, 100% inspection requires rapid imaging of each container. Standard cargo containers are typically 20-50 feet long (6.1-15.2 meters), 8 feet high (2.4 meters), and 6-9 feet wide (1.8-2.7 meters). Larger air cargo containers, which are used to contain a plurality of pieces of luggage or other cargo to be stored in the body of an airplane, may be up to about 240×118×96 inches (6.1×3.0×2.4 meters). MeV radiation sources are typically required to generate radiation with sufficient energy to penetrate through standard cargo containers and the larger air cargo containers. Large collections of objects, such as many pieces of luggage, may also be supported on a pallet. Pallets, which may have supporting side walls, may be of comparable sizes as cargo containers and use of the term cargo container will generally encompass pallets, as well.
A third generation CT system said to be large enough to scan cargo containers is described in U.S. Patent Publication No. 2006/0126772. However, it is believed that such a large third generation CT system would be too expensive to be commercially viable.
In U.S. Pat. No. 5,638,420, large containers are inspected by a system on a movable frame. A source of a fan beam, a cone beam or a pencil beam of X-ray radiation, such as a linear accelerator with an accelerating potential in the MeV range, is mounted on one side of the frame. A detector array is mounted on an opposing side of the frame. The frame, which may be self-propelled, advances across the length of the container during scanning. Radiographic images are generated for analysis by an operator.
In medical CT scanning, there is a configuration referred to as fourth generation, wherein a source of X-ray radiation rotates completely around a patient in a path of a circle within a larger, stationary circular detector array. Fourth generation CT scanners have been found to be an improvement over earlier generations of scanners where both the source and the detector arrays are moved. Scanning times are shorter and the construction of the scanner is simpler. The arrangements of sources and detectors in medical CT scanners are described in more detail in Seeram, Euclid, Computed Tomography: Physical Principles, Clinical Applications, and Quality Control, Second Edition, W.B. Saunders Company, (2001), pp. 10, 77-81, for example. While only the source is moved completely around the patient, enlargement of such a system to accommodate large objects such as cargo containers would still be difficult and expensive.
U.S. Pat. No. 7,103,137, which is assigned to the assignee of the present invention and is incorporated by reference herein, describes a fourth generation type CT system in which one or more sources are moved across an arcuate rail above a cargo container and a plurality of stationary detector modules are arranged in an arc partially below the object. Scanning may be conducted over 180° plus the fan angle, enabling collection of a complete data set for reconstruction.